Add support for offsetting the voting interval in order to bootstrap faster.

A new option TestingV3AuthVotingStartOffset is added which offsets the
starting time of the voting interval. This is possible only when
TestingTorNetwork is set.

This patch makes run_scheduled_events() check for new consensus
downloads every second when TestingTorNetwork, instead of every
minute. This should be fine, see #8532 for reasoning.

This patch also brings MIN_VOTE_SECONDS and MIN_DIST_SECONDS down from
20 to 2 seconds, unconditionally. This makes sanity checking of
misconfiguration slightly less sane.

Addresses #8532.
Hide consensus download interval, depending on TestingTorNetwork, in a macro.
Merge remote-tracking branch 'linus/bug8532'
Rename networkstatus_dl_interval() -> networkstatus_dl_check_interval()